1. Field of the Invention
The present invention relates to a gas discharge lamp, having two electrodes which are connected in gas-tight relationship to the ends of a glass tube, and particularly to such a tube which is used as a flash lamp.
2. The Prior Art
Gas discharge lamps are known, for example, from Philips Technical Review, September 1961, pages 377-404. Such a gas discharge lamp or flash tube consists of a piece of glass tube into which an electrode is placed at each end, in gas-tight relationship with the tube. Typically, the anode consists of tungsten or molybdenum and the cathode consists of a sintered member impregnated with emitting material and getter material, as described, for example, in the German Auslegeschrift 23 32 588. A noble gas such as xenon is typically used to fill the space within the tube. Sometimes an ignition electrode, attached to the exterior, is used to initiate the gas discharge by at least partially ionizing the gas within the tube. The discharge begins at the cathode and spreads toward the anode until the field intensity between the cathode and anode ionizes the space between the cathode and anode and permits the main gas discharge to take place. A gas discharge can also be introduced without an ignition electrode, if a voltage pulse of sufficient magnitude is supplied between the anode and cathode.
Because of the required heat-carrying capacity of the tube, and the need for transparency and electrical insulation, the glass tube typically consists of quartz glass or of a hardened glass such as Pyrex, having a very high melting point, and is preferably formed of borosilicate glass. The metallic feed-through lines which are sealed in gas-tight relationship with the ends of the tube, and which are connected to the electrodes, must be selected so that thermal expansion of the feed-through lines and/or the glass tube does not result in loss of the gas-tight relationship. When hardened glass such as Pyrex is used for the glass tube, tungsten may be used for the electrodes, or at least for the feed-through lines. Glasses which are adapted to the thermal expansion coefficient of tungsten are available in the trade. Quartz glass, however, and some hardened glasses cannot be adapted for use with a tungsten feed-through conductor. When quartz glass or some hardened glasses are used for the glass tube, a connecting piece of intermediate glass must be provided in order to adapt the different thermal expansion coefficients of the quartz or hardened glass and feed-through conductors. When a connecting piece is used, the feed-through conductors may be formed of relatively inexpensive nickel iron instead of the relatively more expensive tungsten.
When a connecting piece is used, it is formed by processing molten glass, such as by melting a glass tube formed of the connecting material onto a feed-through line, or coating the feed-through line by applying the material in melted condition, and assembling the parts with the glass tube by a glass-melting operation. In either case, expensive process steps are required, and the glass materials must be melted at the connecting points in order to obtain a gas-tight connection. In the performance of these processes, not only is a relatively high quantity of energy required, but glass breakage can readily occur, as well as thermally induced stresses within the glass, which can impair the life-span of the gas discharge lamp. Also, when the above-described processes are practiced, it is difficult to control the pressure of the gas within the discharge tube, and it is difficult to closely control the inner electrode spacing within the gas tube.
The disadvantages involved in the use of connecting pieces as described above are overcome by using sintered glass members for sealing the ends of the glass tube.
The sintered glass members can be machine-produced very inexpensively, and their utilization in place of the previously used glass connecting members makes it unnecessary to perform expensive glass-blowing operations during assembly. The precise dimensions of the sintered glass members may be readily controlled, and the feed-through conductors which support the electrodes of the gas discharge lamp are melted into the sintered glass members as they are formed. It is possible to maintain precise spacing of the electrodes of the gas discharge lamp, during assembly, when the sintered glass members are connected to the tubular wall of the lamp. Since the operating life of the lamp, and the light intensity produced thereby, is a function of the electrode spacing, these factors can be controlled quite closely through the use of tubes for the gas discharge lamp which have previously determined fixed lengths, so that there is no waste of the glass material of which the tube is formed, during sealing of the sintered members during assembly of the tube.